Most fuel cells presently being studied are polymer electrolyte membrane (PEM)-based systems. A membrane, such as NAFION®, separates the cathode and anode compartments, preventing fuel from diffusing toward and reacting at the wrong electrode. At the same time this PEM allows protons (H+) to diffuse from the anode to the cathode. The cathode and anode electrodes are formed by applying catalyst (such as Pt or alloys of Pt) to both sides of the membrane. The resulting catalyst-covered membrane is referred to as the membrane-electrode assembly (MEA).
Hydrogen has been the preferred fuel for fuel cells. In order to obtain lightweight micro fuel cells with high specific energy densities, hydrogen gas must be stored under high pressure, which requires a special container, and safety issues related to pressure and flammability have to be taken into account [8-11].
Safer high energy density fuels are certain hydrocarbons such as methanol and formic acid, since they can be stored in liquid form under ambient conditions. Hence, most studies toward micro fuel cells focus on the development of microfluidic direct methanol fuel cells (μ-DM-FCs) [4,12-14], and direct formic acid fuel cells (μ-DFA-FCs) [5-7,15]. While the performance of these μ-DM-FC and μ-DM-FA prototypes is promising, several technical issues associated with their development and operation (e.g. fuel crossover, membrane dry out, and cathode flooding) remain [16].
The reaction kinetics of both fuel oxidation and oxygen reduction are known to be better (i.e. faster) in alkaline media than in acidic media. Better kinetics can immediately lead to better performance. In addition, the chemistry of fuel cells in alkaline media allow the use of less precious materials as the catalysts (e.g. Ni or Ag instead of Pt) [44-46]. In alkaline media the fuel oxidation and oxygen reduction reactions are slightly different: at the cathode hydroxide ions are formed during the electro-reduction of oxygen. These hydroxide ions diffuse through the PEM towards the anode. At the anode during the electro-oxidation of fuel these hydroxide ions are consumed.
Presently, almost all PEM-based fuel cells are run in acidic media. It has proven difficult to obtain alkaline media compatible membranes that are (i) capable of transporting hydroxide ions (OH−), and (ii) capable of preventing fuel from crossing over to the cathode (fuel crossover), a process detrimental to fuel cell performance. In addition, carbon dioxide (CO2) forms at the anode which reacts with OH− to produce poorly soluble carbonate ions (CO32−), eventually resulting in precipitation of carbonate salt [44-46]. This quickly leads to clogging of the pores of the membrane-electrode assembly. Therefore, alkaline fuel cells use hydrogen to avoid these problems.
There are several types of fuel cells that do not have a PEM. Laminar flow fuel cells use parallel laminar flow between two streams of liquid to create an interface between the streams, which replaces the PEM or salt bridge of conventional devices [48,49]. When the first stream, containing an oxidizer, comes into contact with the first electrode, and the second stream, containing the fuel, comes into contact with the second electrode, a current is produced, while charge migration from the anode to the cathode occurs through the interface. This cell design minimizes crossover by maximizing consumption of the fuel before it diffuses into the oxidant stream. When run as an electrolytic cell (rather than as a galvanic cell), these microfluidic devices can be used for synthesis [47].
A different micro fuel cell concept that also lacks a PEM has been reported [10]. Delivery of fuel (hydrogen) and oxidant (oxygen) relies on diffusive transport through a permeable polydimethylsiloxane (PDMS) membrane that is placed on both the anode and cathode, while the anode and cathode compartments are connected via tiny microfluidic channels. The high permeability of PDMS for oxygen and hydrogen provides a significant advantage in overcoming mass transfer limitations.
Biofuel cells, having immobilized enzymes and mediators on the electrode, may also avoid the need for a PEM. Here, the specificity of the enzymes prevents reaction of the fuel at the wrong electrode. A flaw with these systems is that two different enzymes, one at the anode and the other at the cathode, are used in a common liquid media, so that the pH of the system is selected to optimize the overall system, and is typically suboptimal for one or both of the enzymes [41].